I've been reading throught the Apollo 11 Flight Journals and was wondering how they were originally recorded and transcribed. It seems that they were painstakingly accurate down to the last second that something was said in terms of MET. I was honestly wondering how they got time-stamps down to seconds, are those just guesses or was even radio silence recorded, then reverse engineered later? Obviously the MP3 format had not existed, does anyone know what technologies were used to capture and record the transmissions to and from Apollo 11 in comparison to how we'd do it today?

EDIT: I was assuming it would be a device located at the command center, but because of the transmissions being recorded even when L.O.S.'d I was thinking I was incorrect. I was also assuming that having a recorder in both the CSM and LEM would be unneeded weight (audio is transcribed in both the LEM and CSM).

Here's a fun excerpt from them reading the "daily news" to the crew on Apollo 11 illustrating how detailed these logs actually were (note the accuracy of time-stamps):

072:34:02 McCandless: You might be interested in knowing, since you are already on the way, that a Houston astrologer, Ruby Graham, says that all the signs are right for your trip to the Moon. She says that Neil is clever, Mike has good judgment, and Buzz can work out intricate problems. She also says Neil tends to see the world through rose-colored glasses, but he is always ready to help the afflicted or distressed. Neil, you are also supposed to have, quote, intuition that enables you to interpret life with feeling, unquote. Buzz is to be very sociable and cannot bear to be alone in addition to having excellent critical ability. Since she didn't know at what hour Mike was born, she has decided that he either has the same attributes as Neil or he is inventive with an unconventional attitude that might seem eccentric to the unimaginative.

072:34:56 Haise: And last but not...

072:34:58 Collins: Who said all that?

072:35:00 McCandless: [Laughter] Ruby Graham, an astrologer here in Houston. Now we check with Flight Operations for all the signs for the mission, and then we, of course, had to make sure that everything was really all set. [Long pause.]

Another example of a line with EXTREME data density, I was wondering how they could've possibly kept all of these numbers correct in a transcript/recording in real-time:

072:53:54 Armstrong: Roger. LOI-1, SPS/G&N: 62710, plus 0.98, minus 0.19; 075:49:49.65; minus 2889.7, minus 0394.4, minus 0068.6; 358, 226, 347; 0169.2, plus 0061.0; 2917.3, 6:02, 2910.8; 31, 106.6, 35.8. GDC align, Vega and Deneb, 243, 183, 012. No ullage. Horizon in the hatch window 2 minutes before TIG. AOS with an LOI, 76:15:29; AOS without an LOI, 76:05:30. HA before the burn, 431.3; HP, minus 128.2. Say again LOS time.

It seems that even when not in contact with Huston (on the far side of the moon) the audio is still being both transcribed and recorded too:

[At 075:41:23, precisely when Houston predicted, the radio signal from Apollo 11 is lost.]

075:41:23 Aldrin (onboard): How soon are we going to...

075:41:24 Armstrong (onboard): [Garble] take 2.

075:41:25 Aldrin (onboard): 1 second early. Okay. Main Bus Ties...

075:41:28 Armstrong (onboard): Okay.

075:41:29 Collins (onboard): I'm going to turn my S-Band Volume down, so you can [garble].

075:41:33 Armstrong (onboard): Down Voice Backup.

075:41:39 Aldrin (onboard): Main Bus Tie A coming On. Have you got TVC Gimbal Drive - Pitch and Yaw to Auto, huh?

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    $\begingroup$ That would have been trivial to do with an audio recorder (e.g. open-reel tape recorders were well established by then) and a clock. Record on a schedule, then transcribe at your leisure. $\endgroup$
    – Hobbes
    Jul 2, 2018 at 18:55
  • $\begingroup$ @Hobbes ah, so it was an onboard tape-recorder then? Nothing special about it and I'm just overthinking it? I was assuming it would be a device located at the command center, but because of the transmissions being recorded even when L.O.S.'d I was thinking I was incorrect. I was also assuming that having a recorder in both the CSM and LEM would be unneeded weight (audio is transcribed in both the LEM and CSM). $\endgroup$ Jul 2, 2018 at 19:00
  • $\begingroup$ Both onboard CSM and LM and earth side tape recorders were in use; the weight of the equipment was well worth the scientific, historical, and technical value. $\endgroup$ Jul 2, 2018 at 20:10

2 Answers 2


Besides the recording equipment Earth-side, which picked up the "air-to-ground loop" (i.e. voice transmissions between the spacecraft and mission control), both the CM and the LM had multitrack tape recorders which were used to store and relay both telemetry and onboard voice data. The CM system was called DSE (Data Storage Equipment) and the LM system was DSEA (Data Storage Electronics Assembly).

Apollo reel-to-reel DSE recorder

The DSE recorder weighed about 30 pounds -- a substantial payload investment, but the voice recording data from the mission had substantial historical and scientific value, as well as its value in analyzing any problems that cropped up during each flight.

During the mission, data from the CM DSE system was transmitted back to Earth -- when a reel of tape was filled, it could be rewound onboard and downloaded ("dumped") by mission control at high speed. I don't think it had the capability to have one reel recording while another was downloading; there are many gaps in the transcripts of the on-board recordings.

Since the on-board recordings were on a multichannel system, timestamps could be recorded along with telemetry data on parallel tracks, making it possible to reliably timestamp the transcripts.

The transcription itself would be done the old-fashioned way, with a human listening to a tape; they likely had something set up to display the timestamp of the current tape position, and I would assume they could pause and rewind as needed to get the details of a particularly complex call. I have seen occasional obvious errors in the transcripts, e.g. a call from the lunar surface being attributed to the command module pilot; it's likely that there are many less obvious errors such as numerical transpositions or homophonic misinterpretations.

The ability to download onboard recording data during the mission was used on occasion to transmit sensitive information to mission control without it going over the air-to-ground voice loop. On Apollo 8, mission commander Frank Borman was ill with some sort of stomach bug during the outbound flight; they discussed his symptoms aboard the CM, then hinted to mission control that they ought to download and review their tapes. This strategy actually wasn't very effective; the crew's hint was too subtle.

Around 5 hours, 44 minutes into the flight, one of the crew members says:

SC "Houston, we've rewound the tape, you can dump it at your convenience."

And according to Jeffrey Kluger's "Apollo 8", written with Apollo 8 CMP Jim Lovell's help, this was the "hint". It's not until 19 hours later, a full 24 hours into the flight, that they hint a little harder:

SC "How've you been reading our tape dumps?"

CAPCOM "Stand by one, Frank.... I'll check on your tape dump."

CAPCOM "Apollo 8, Houston, the quality of the tape dumps has been very good."

SC "How's the voice quality been?"

CAPCOM "It's been very good, Frank."

SC "Okay. We'll send you something new here shortly."

Eventually mission control did listen to enough of the voice recordings to realize what was up, but by the time they caught on, Borman was already feeling better.

The LM's DSEA system was more limited. In its earliest versions, it recorded continuously rather than being voice-activated, and had only a 10 hour recording capacity, shorter than the LM's mission life. Nor did it have the real-time download capability of the CM DSE; a cartridge containing the recording tape could be removed and returned to the CM after leaving the moon.

NASA's "history portal" site often provides separate transcriptions of the air-to-ground loop, mission commentary, and onboard recording, as with this Apollo 11 collection: https://www.jsc.nasa.gov/history/mission_trans/apollo11.htm

Some of the Apollo flight journal annotated transcripts combine the onboard and air-to-ground recordings, color-coding the two distinctly; those give the most complete picture of events aboard the spacecraft.

  • $\begingroup$ That's pretty cool that they could relay the private tape recordings back down to earth on demand. Thanks- helpful insight 🙂. $\endgroup$ Jul 2, 2018 at 21:14
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    $\begingroup$ Regarding Borman’s ”flu” on Apollo 8: It wasn’t a real flu but probably gastroenteritis or just space sickness. $\endgroup$
    – Michael
    Jul 3, 2018 at 9:53
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    $\begingroup$ And probably not space sickness, since he was, ah, venting at both ends. $\endgroup$ Jul 3, 2018 at 14:14
  • $\begingroup$ @RussellBorogove Oh gosh, you have the best euphemisms for... biological emergencies in space. That one had me laughing. $\endgroup$ Nov 30, 2018 at 20:54

Much of the Flight Journals contains radio communications between the spacecraft and ground control. They were recorded at the control center and transcribed.

With kind help from NASA's History Office, the Apollo Flight Journal was prepared from Technical and Public Affairs Office transcripts made at the time. These were converted to electronic text and checked, where possible, with audio recordings of the air ground communications.

That leaves the time slices where Apollo was behind the Moon and there was no radio contact with the ground. Given the amount of detail there (check the end of that transcript), it looks like those were recorded too.

Edit: The tape recorder below was used on the Apollo missions, but this wasn't the primary source for those transcripts. That would be the DSE as described in Russell's answer.

enter image description here

Apollo astronauts’ carried a small, battery-powered cassette voice recorder with them in the Command Module during lunar missions. The recorder was used to record information relevant to the crew log. This recorder and tapes were flown on the Apollo 12 mission and transferred to the Smithsonian in 1974.

It's a Sony TC-50. And another link.

  • $\begingroup$ Neat! I wonder if they were instructed to only record during periods of isolation from command or what the general protocol was. I dont know how many hours a cassette can record, but there had to have been some limits. Thank you ☺. $\endgroup$ Jul 2, 2018 at 19:51
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    $\begingroup$ The CM at least had an integral voice recorder as part of the data storage equipment (DSE) system. I know portable cassette decks like that were used for the crew’s entertainment; I don’t think they were the primary recording device. The DSE recordings could be downloaded to Earth during the mission (possibly at high speed?) without being played over the main air to ground loop, which allowed the crews to semi-privately discuss health issues in a couple of casss. $\endgroup$ Jul 2, 2018 at 20:17
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    $\begingroup$ @MagicOctopusUrn The TC-50 used the standard 'Compact Cassette', so 2x45 minutes per cassette. I know, longer tapes became available eventually, but not sure they were there back then and C-120 is notoriously less reliable than C-90 so unlikely to be used by NASA. $\endgroup$
    – Hobbes
    Jul 3, 2018 at 13:43
  • $\begingroup$ I wonder if they had special radiation shielding on a pouch for them to store them in. Going to guess cassette tape is highly susceptible to radiation, though I don't actually know. $\endgroup$ Jul 3, 2018 at 17:19
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    $\begingroup$ I wouldn't expect non-lethal amounts of radiation to do much harm to analog cassette tape. $\endgroup$ Jul 3, 2018 at 19:19

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